Aftershock Sequence Unusual, Seismologists Say

Last night's magnitude 6.1
aftershock in Fiordland came as a mild surprise,
seismologists said today.

" After the August 22 magnitude
7.1 earthquake in Fiordland, we anticipated getting an
aftershock of about magnitude 6," said seismologist Warwick
Smith of the Institute of Geological and Nuclear Sciences
Limited (GNS).

" A magnitude 6.2 aftershock occurred
within two hours of the main shock. But it's unusual to get
two aftershocks of roughly this size in an aftershock
sequence," Dr Smith said.

No two aftershock sequences were
exactly the same, but they generally followed a commonly
observed pattern.

" The rule of thumb is that the biggest
aftershock is generally about one unit magnitude smaller
than the main shock. It's rare to see two aftershocks of
this size in a sequence.

" It's not possible to attach
any significance to this aftershock sequence other than it's
slightly unusual."

Aftershocks were expected to continue
in Fiordland for some weeks, or even months. But they would
gradually become less frequent.

The biggest aftershock in
the Fiordland sequence is still the magnitude 6.2 jolt that
occurred two hours after the main shock. Since August 22,
the have been 19 aftershocks of magnitude 5 or
greater.

Within two days of the main shock on August 22,
GNS seismologists had deployed eight portable seismometers
near the epicentre at Secretary Island, at the entrance to
Doubtful Sound, to record aftershocks.

Close study of
the aftershocks would reveal valuable information about the
size and orientation of the "fault break" in the crust, and
indicate if the main shock and aftershocks had affected
any nearby faults. ENDS

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